The primary role of the Mouse Genetics Core Facility (MGCF) is to facilitate the use of mouse molecular genetics at MSKCC for in vivo studies of gene functions germane to cancer. Relevant fields where mouse models can be applied include cell growth and behavior, cellular differentiation, embryonic development, immunobiology, genome integrity, and malignant transformation. The Core consists of 3 groups: the Transgenic Mouse Group, the Colony Management Group, and the ES Cell Culture Group. Together they provide the following services: (1) Production of transgenic mice: transgene DNA purification, pronuclear injection, genotyping of founder mice, and breeding of positive founders to provide G1 progeny. (2) Gene Targeting: electroporation of gene targeting vector into ES cells, selection and identification of clones, and cryopreservation of targeted ES cell clones. (3) Production of gene targeted mice: generation of chimeric mice by blastocyst injection and identification of germline chimeras. (4) Long term storage of transgenic, gene targeted, congenic, and other mutant mouse strains by cryopreservation of sperm or embryos. (5) Rederivation by embryo transfer or IVF for strain importation and recovery. (6) Performance of specialized animal surgical procedures and embryological techniques. (7) Provision of specialized mouse strains/lines for investigators' research. (8) Provision of husbandry service to assist investigators in the maintenance of transgenic, gene targeted, and mutant strains of mice. (9) Comprehensive management of transgenic, gene targeted, and mutant mouse colonies for investigators. (10) Genotyping of transgenic, gene targeted, and mutant mouse lines for users. (11) Assistance in all aspects of ES cell culture and ES cell establishment from transgenic, gene targeted, and mutant mice. (12) Consultation & training in mouse genome manipulation and mouse genetics. In the past several years, the promotion of translational research at the Center and the rapid advancement of genomic technologies, had led to a dramatic increase in the number of laboratories using mouse models in their research. To meet these demands, the Core has implemented four new services, hired additional personnel, and re-organized the staff so that transgenic and gene targeted mice can be efficiently produced. Furthermore, the Core now offers more comprehensive services to aid investigators not just in creating the animal models of interest but also in maintaining, cross-breeding, and utilization of the lines as experimental models.